1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a solid core and a cover and having dimples on a surface thereof.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. By using a core having an outer-hard/inner-soft structure, a low spin rate and a high launch angle can be achieved.
JPH2-264674 (U.S. Pat. No. 5,072,944) discloses a golf ball with a core consisting of a center core and an outer layer. The center core is flexible, and the outer layer is hard. The core suppresses a spin rate.
JPH6-98949 (U.S. Pat. No. 5,516,110) discloses a golf ball having a constant hardness between: a point that is located at a distance of 5 mm from a central point; and a point that is located at a distance of 10 mm from the central point. A similar golf ball is also disclosed in JPH6-154357 (U.S. Pat. No. 5,403,010).
JPH7-112036 (U.S. Pat. No. 5,562,287) discloses a golf ball having a small difference between a central hardness and a surface hardness of a core. The core contributes to the resilience performance of the golf ball.
JP2002-764 (US 2002/032077) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2002-765 (US 2002/019269).
JP2003-33447 (US 2003/032501) discloses a golf ball with a core for which a rubber composition includes a polysulfide. The polysulfide contributes to the resilience performance of the golf ball.
JP2008-194473 (US 2008/194357, US 2008/312008) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2010-22504.
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.
There have been various proposals for the shapes of dimples. U.S. Pat. No. 7,250,012 discloses a golf ball that has dimples each having an annular tubular portion.
JP2001-54592 (U.S. Pat. No. 6,558,274) discloses a golf ball that has first dimples and second dimples. The second dimples are recessed from the first dimples.
JP2002-531232 (U.S. Pat. No. 6,162,136) discloses a golf ball that has dimples each having a central depression, a land ring, and an annular depression.
JP2003-290390 (US 2003/190968) discloses a golf ball that has dimples each having a projecting bottom. The curvature radius of the bottom is large.
JP2008-12300 (US 2008/004137) discloses a golf ball that has dimples each having a projection. The projection is surrounded by a ring-shaped recess.
In the golf ball disclosed in JPH2-264674, the structure of the core is complicated. The core produces an energy loss when being hit. In addition, the core has inferior durability.
In the golf ball disclosed in JPH6-98949, a range where the hardness is constant is narrow. The golf ball has inferior resilience performance. Similarly, the golf ball disclosed in JPH6-154357 also has inferior resilience performance.
In the golf ball disclosed in JPH7-112036, a spin rate is excessive. The golf ball has a small flight distance.
The golf ball disclosed in JP2002-764 has inferior resilience performance. Similarly, the golf ball disclosed in JP2002-765 also has inferior resilience performance.
In the golf ball disclosed in JP2003-33447, a spin rate is excessive. The golf ball has inferior flight performance.
In the golf ball disclosed in JP2008-194473, there is a zone in which a hardness decreases from the central point of the core toward the surface of the core. The golf ball has inferior resilience performance. In the golf ball, a spin rate is excessive. The golf ball has inferior flight performance. Similarly, the golf ball disclosed in JP2010-22504 also has inferior flight performance.
The flight performance of the golf balls disclosed in U.S. Pat. No. 7,250,012, JP2001-54592, JP2002-531232, JP2003-290390, and JP2008-12300 is not sufficient. There is room for improvement in the conventional dimples.
An object of the present invention is to provide a golf ball having excellent flight performance.